ABSTRACT Social bonds are central to our experience, shaping both our health and happiness. Few of these bonds are as important to well-being as one?s relationship with a partner. Because social bonds play such an important role in human lives, studies of neural mechanisms of adult attachments in animal models have a high translational value. Most commonly used laboratory models, however, do not form strong and enduring attachments. For this reason, the socially monogamous prairie vole, in which males and females form stable pair-bonds and raise young together, have become an increasingly important species in social neuroscience. Work on prairie voles, for example, has revealed how the neuropeptides oxytocin and vasopressin help bring about pair bonds by modulating the activity of subcortical reward regions. Although vole research has led to human studies that examine reward circuitry in the context of attachment, most work in human social neuroscience is focused on how cortical structures contribute to social cognition, an area that is understudied in animal models. Motivated by this gap between human and animal studies, we propose to develop and apply modern systems neuroscience tools for the automated and unbiased analyses of brain structure and function in the prairie vole. Our aim is to comprehensively map the diverse circuits that are modified during pair-bond formation, and to examine how these circuits are used during the expression of bonds. In Aim 1, we will develop a detailed three-dimensional prairie-vole brain atlas, and integrate it into a computational pipeline for the automated whole-brain imaging of neuronal cell types, long-range projections, and synaptic densities. We will then use these methods to map structural differences between mouse and prairie vole brains, between male and female prairie voles, and between bonded and un-bonded voles. In Aim 2, we will use our whole-brain approach to map brain immediate-early gene induction (including c-fos and alternatives) across a 24h interval of pair-bond formation. We follow this by identifying circuit activity associated with the selective recognition of a partner, or with the discrimination between a partner and stranger. These experiments will identify the substrates of bond formation, and will clarify how these brain regions interact with other circuits during the expression of selective attachment. Finally, in Aim 3, we will study how one specific cortical region, the retrosplenial cortex (RSC), contributes to the formation and expression of pair-bonds. This work follows a growing body of data implicating the RSC in long-term memory, human social cognition and prairie-vole bonding. The study will use AAV-based chemogenetic manipulations of the RSC to investigate its contribution to whole-brain activity patterns and prairie vole bonding.